test: add tests for to_hashable_items, variant_distance, and split_haplotypes

[ameynert]

Summary

Adds tests/test_haplotype.py covering the three functions in haplotype.py that don't require reference genome sequence data:

• to_hashable_items: empty dict, single entry, sort-by-key order
• variant_distance (requires hail_context): adjacent SNPs (distance 0), SNPs with a gap, deletion that closes the gap
• split_haplotypes (requires hail_context): no split when all variants fit in one window; split at a gap exceeding the window; singleton segment discarded after a split

The variant_distance and split_haplotypes tests use the session-scoped hail_context fixture and a _make_haplotype_table helper that builds a minimal Hail table with a tstruct schema (position-only locus, alleles, haplotype, gnomad_freqs).

Test plan

• pixi run check-all passes
• All 9 tests in test_haplotype.py pass

🤖 Generated with Claude Code

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch am_14_test_haplotype

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